In a wireless communication system, the output of a power amplifier (Power Amplifier, PA) is generally required to have very high linearity to meet the stringent requirements of air interface standards, but a linear PA is inefficient and very expensive, and an efficient nonlinear PA will produce third-order, fifth-order, seventh-order or other intermodulation components, which will interfere with adjacent channels. In order to improve the output efficiency of the PA and reduce the cost as much as possible, the nonlinear characteristics of the PA must be calibrated, and performing predistortion processing on the input signal of the PA is a good choice.
The essence of digital predistortion (Digital Predistortion, DPD) is to perform preset anti-distortion on the amplitude and phase of the input signal of the power amplifier in advance to counteract the nonlinearity of the power amplifier. The specific implementation manner is as follows: comparing a feedback signal y(n) of the power amplifier with a forward transmission signal z(n), obtaining a group of coefficients by reasonably modeling the power amplifier to approximately characterize the nonlinear characteristics of the power amplifier, and then performing nonlinear predistortion on a transmission signal x(n) via the coefficients and the model, such that the power amplified output signal of a pre-distorted signal is linear amplification of an original transmission signal, y(n)=Gx(n).
Since the power amplifier model in the exiting DPD algorithm generally adopts a memory polynomial (Memory Polynomial MP) model, a mathematical model of the MP model for performing signal predistortion processing is:
                                                                        z                ⁡                                  (                  n                  )                                            =                            ⁢                                                ∑                                      m                    =                    0                                    M                                ⁢                                                      x                    ⁡                                          (                                              n                        -                        m                                            )                                                        ⁢                                                            ∑                                              k                        =                        1                                            K                                        ⁢                                                                  w                                                  m                          ,                          q                                                                    ⁢                                                                                                                              x                            ⁡                                                          (                                                              n                                -                                m                                                            )                                                                                                                                                          (                                                      k                            -                            1                                                    )                                                                                                                                                                                            =                            ⁢                                                ∑                                      m                    =                    0                                    M                                ⁢                                                      x                    ⁡                                          (                                              n                        -                        m                                            )                                                        ·                                                            LUT                      m                                        ⁡                                          (                                                                                                x                          ⁡                                                      (                                                          n                              -                              m                                                        )                                                                                                                      )                                                                                                                                              =                            ⁢                                                ∑                                      m                    =                    0                                    M                                ⁢                                                      x                    ⁡                                          (                                              n                        -                        m                                            )                                                        ·                                                            LUT                      m                                        ⁡                                          (                                              Q                        ⁡                                                  (                                                      r                                                          n                              ,                              m                                                                                )                                                                    )                                                                                                                                                              wherein        ⁢                                  ⁢                                                            LUT                m                            ⁡                              (                                                                        x                    ⁡                                          (                                              n                        -                        m                                            )                                                                                        )                                      =                                          ∑                                  k                  =                  1                                K                            ⁢                                                w                                      m                    ,                    k                                                  ⁢                                                                                                x                      ⁡                                              (                                                  n                          -                          m                                                )                                                                                                                      (                                          k                      -                      1                                        )                                                                                ,                                          ⁢                      m            =                          1              ⁢                                                          ⁢              …              ⁢                                                          ⁢              M                                ,                      n            =                          1              ⁢                                                          ⁢              …              ⁢                                                          ⁢              N                                ,                                    k              =                              1                ⁢                                                                  ⁢                …                ⁢                                                                  ⁢                K                                      ;                                                          K refers to a nonlinear factor and is a natural number, and the value is 3-7; M refers to a memory depth and is a natural number, and the value is 3-6; N refers to the number of sampling points and is a natural number, and the value is generally 4096-16384.
z(n) refers to a forward sending signal obtained after predistortion processing of the input signal; Q(•) refers to a quantization factor, rn,m refers to the amplitude of the input signal: rn,m=|x(n−m)|; LUTm(|x(n−m)|) refers to a corresponding predistortion parameter with the input signal amplitude |x(n−m)| as an index, and the input address of the LUTm(|x(n−m)|) is determined according to the quantified amplitude Q(rn,m) of the input signal; Wm,k refers to a predistortion coefficient calculated by DPD adaptive filtering; after calculating the Wm,k, a predistortion module may calculate a predistortion signal y(n). When performing predistortion update, a DPD module stores a predistortion parameter according to |x(n−m)|, and extracts the predistortion parameter according to |x(n−m)| during predistortion processing. After each DPD coefficient calculation, all DPD parameters stored in an LUT parameter list will be updated once. Specifically, after the predistortion coefficient is calculated, the updated LUT parameter list is obtained by means of the following algorithm:
                    LUT        m            ⁡              (        r        )              =          A      ·                        ∑                      k            =            1                    K                ⁢                              w                          m              ,              k                                ·                                    (                              r                ·                Q                            )                                      (                              k                -                1                            )                                                      r      =      1        ,          …      ⁢                          ⁢      R                  m      =      1        ,          …      ⁢                          ⁢      M      wherein, R refers to the length of the LUT parameter list stored by a primary memory factor, for example, is generally 256, 512 or the like. The quantification factor Q is equal to the maximum signal /R capable of being stored by the system, for example, Q=32768/R, if R=512, then Q=64.
The above-mentioned model may well respond to the nonlinearity of the power amplifier under general conditions. But for a wideband long term evolution (Long Term Evolution, LTE) 40 MHz system or a wider system, the left and right difference in adjacent channel power ratio (Adjacent Channel Power Ratio, ACPR) is large and the effect is poor. Because signals adopted in wideband DPD technology are high speed sampling signals, the requirements on delay are very high and good accuracy is needed. Moreover, with the change of time and temperature, the power amplifier devices will jitter due to delay, and even if very small jitter will generate a relatively large error in the predistortion processing.
In the prior art, it is proposed to correctly extract a nonlinearly distorted delayed synchronous loop circuit overlapped on the output of a digital predistortion wireless transmitter, in order to achieve synchronous transmission and reception. However, after predistortion synchronization, whether to perform further out-of-step processing and calibration on the synchronized signal is not mentioned, thus the DPD effect could not be improved by delay.
Since the input signal is usually a low speed baseband signal, but the digital predistortion needs to work at a high rate, interpolation filtering needs to be performed on the input baseband signal to improve the rate of the input signal, in order to get a better predistortion effect, the input signal generally needs to be interpolated to improve the rate of the input signal, for example, the interpolation multiple of LTE-time division duplexing (Time Division Duplexing, TDD) is 48, and the intermediate frequency rate is 245.76 MHz, but a problem is brought that the delay is required to be very accurate and is better to be accurate to a clock cycle. On one hand, the round-trip time of a link is very difficult to measure in the prior art, on the other hand, even if the round-trip time may be accurately measured, with the change of time and temperature of the devices, the delay will jitter, and even if very small jitter will generate a relatively large error in the predistortion processing.